Process for preparing oximosilanes

ABSTRACT

Described is a process for producing oximosilanes having the general formula (I) 
     
         R.sup.1 a Si(O--N═Cr.sup.2 R.sup.3).sub.4-a            (I) 
    
     wherein R 1  represents a straight-chain or branched alkyl residue having between 1 and 18 carbon atomns, a straight-chain or branched alkenyl residue having between 2 and 18 carbon atoms, an aryl or aralkyl residue optionally substituted by halogens, nitro groups, amino groups and/or alkyl residues having between 1 and 4 carbon atoms, or a straight-chain or branched alkoxy-residue having between 1 and 8 carbon atoms, a is an integer of 0 to 3, and R 2  and R 3 , independently of each other, represent straight-chain or branched alkyl residues having between 1 and 18 atoms, optionally substituted by halogens, nitro groups, amino groups and/or alkyl residues having between 1 and 4 carbon atoms, by reacting an acyloxisilane with an oxime in molar excess, characterized in that an acyloxy-silane of general formula (II) 
     
         R.sup.1 a Si(OCOR.sup.4).sub.4-a                           (II) 
    
     wherein R 1  and a have the above mentioined significations and R 4  represents straight-chain or branched alkyl residues having between 1 and 18 carbon atoms, preferably between 2 and 8 and more preferably between 2 and 4 carbon atoms, straight-chain or branched alkenyl residues having between 2 and 18 carbon atoms or aryl or aralkyl residues optionally substituted with halogens, nitro groups, amino groups and/or alkyl residues having between 1 and 4 carbon atoms, with a ketoxime having the general formula (III) ##STR1## wherein R 2  and R 3  possess the above significations, in the absence of an acid acceptor, while distilling off the carboxylic acid, which is released from the acyloxisilane of the general formula (III).

The subject of the present invention relates to the production ofoximosilanes by a rapid, economical and safe method that obviates theuse of solvents.

It is known that oximosilanes can be produced by inducing a reaction oforganochlorosilanes with oximes in the presence of suitable acidacceptors and solvents (U.S. Pat. Nos. 3,962,160, 3,441,583, 3,341,486,3,817,909, DE-AS 13 01 140, DE-AS 11 20 690, DE-AS 12 55 924, FR-PS 1118 495, EP-PS 0 036 262, W. Noll, Chemie und Technologie der Silikone,Seite 342, Verlag Chemie, Weilheim 1968).

The byproducts resulting from the use of this known method will be,depending upon the acid acceptor employed, quaternary ammonium saltssuch as ammonium chloride or amine hydrochloride. Due to the inevitableformation of such salts during the reaction, it is necessary, in orderto be able to produce an acceptable crystal paste, that a large quantityof solvent be added to act as a surfactant. After the reaction hasended, the resulting oximosilane is separated from the salt formedduring the reaction, and the salt then washed with large quantities ofsolvent. The fact that these salts cannot be economically washed awaygives rise to the great dsadvantage that a considerable quantity of thedesired end product, i.e. oximosilane, is absorbed onto their surface.

The desired oximosilane, which is contained in the filtrate, remains inthe bottoms after the solvent is driven off. Subsequent distillation ofthe oximosilane yields the pure end product.

It has been shown, in practice, that residues containing such salts andpresent in the oximosilane bottoms can, during the subsequentdistillation procedure, cause the explosive disintegration of theoximosilane through a Beckmann rearrangement. This phenomenon has beentreated in the report of Leslie J. Tyler, Vice President and ResearchDirector of Dow Corning Company, (published in C+E News, issue of2/9/1974).

The literature describes a number of processes that avoid the formationof such salts. In this vein, DE-OS 27 19 008 sheds light on the reactionof a substituted aminosilane, e.g. ethyl tri(cyclohexylamino) silane ormethyl-tri (secondary butyl-amino) silane with methyl ethylketoxime,whereafter the thus formed cyclohexylamine or secondary butylamine isdistilled off so as to produce a ketoximosilane, whereby the residualdistillate, the oximosilane, can be used without further treatment toproduce coloured or transparent silicon sealants. Such a process doesnot, however, solve the present problem, because the offending salts,already issuing from the production of the aminosilanes, renders thismethod, too, uneconomical.

Described in JP-PS 80-016008 (C.A. (1980), 92-199928) is the reaction ofalkoxisilanes, preferably silicontetraalkoxisilanes aided by atransesterification catalyst from the cyclo-diaza undazene series, withketoximes. It has been shown, however, that the rather sluggish advanceof such reactions coupled with the formation of a plurality of differentsubstitute products causes such reactions to be extremely slow. Suchreactions, moreover, being ineffective facilitators of alkoxi groupreplacement, render the corresponding process incapable of providing aneconomical solution to the difficulties related to the production oftetra and trifunctional oximosilanes.

U.S. Pat. Nos. 3,697,568 describes iminoxiorganosilanes and a processfor their production. Such iminoxisilanes distinguish themselves fromthe substance produced according to the invention in that they featurenot only the oxime residue but also an additional glycol-ether residue.In order to produce such compounds, either a halogen silane or anacyloxisilane is converted with an oxime in the presence of ammonia or aprimary alkylamine or arylamine in an inert diluent (claims 12 and 13).As illustrated by the examples given, the use of a nitrogen-containingbase in an inert diluent for the purpose of enabling the reaction tocompletely run its course, is an unavoidable necessity. Example 6 ofthis publication describes the reaction of methyltriaceto-oxisilane withan acetaldehydeoxime, in which acetic acid and volatile products arefirst distilled off, and then to facilitate the reaction with themonomethylether of ethylene glycol in the presence of ammonia, isbrought to completion.

After this reaction has run its course, the ammonium salt-containingbottom is, as in other similar processes, filtered out and washed withan organic solvent. This known process also produces large quantities ofquaternary ammonium salts that, as has been demonstrated above, are ableto cause, during the subsequent distillation of the solvent or the endproduct, an explosive disintegration of the oximosilane due to Beckmannrearrangement.

The objective of the present invention is thus the identification of aprocess which, while it does not involve the use of solvents, is capableof producing the required oximosilanes without engendering harmfulsolids, i.e. quaternary ammonium salts, such a process being assisted bya relatively rapid and economical method and thereby being capable ofavoiding the explosive disintegration of the oximosilane throughBeckmann rearrangement.

This objective has been shown to be attainable if an acyloxisilane iscaused to react with a ketoxime without the use of an acid acceptor andif the carboxylic acid released from the acyloxisilane is removed bydistillation.

The objective of the invention is thus the creation of a processsuitable for producing oximosilanes of the general formula (I)

    R.sup.1.sub.a Si(O--N═CR.sup.2 R.sup.3).sub.4-a        (I)

wherein

R¹ represents a straight-chain or branched alkyl residue having between1 and 18 carbon atoms, a straight-chain or branched alkenyl residuehving between 2 and 18 carbon atoms, an aryl or aralkyl residuesubstituted with halogens, nitro groups, amino groups and/or alkylresidues having between 1 and 4 carbon atoms, or a straight-chain orbranched alkoxy residue having between 1 and 8 carbon atoms, a is aninteger of 0 to 3 and

R² and R³ represent, independently of each other, straight-chain orbranched alkyl residues having between 1 and 18 carbon atoms, optionallysubstituted with halogens, nitro groups, amino groups and/or alkylresidues having between 1 and 4 carbon atoms by reaction of anacyloxysilane with an oxime in molar excess, characterized in that anacyloxisilane of the general formula (II)

    R.sup.1 a Si(OCOR.sup.4).sub.4-a                           (II)

in which R¹ and a have the above significations and R⁴ representsstraight-chained or branched alkyl residues having between 1 and 18carbon atoms, straight-chain or branched alkyl residues having between 2and 18 carbon atoms or aryl or aralkyl residues optionally substitutedwith halogens, nitro groups, amino groups and/or alkyl residues havingbetween 1 and 4 carbon atoms, is reacted with a ketoxime having thegeneral formula (III) ##STR2## wherein R² and R³ have the abovesignifications, without the use of an acid acceptor, and whereincarboxylic acid(s) released from the acyloxisilane having the generalformula (III) is removed by distillation.

Such a process, which does not require the use of solvents, allows ahigh rate of recovery of a desired oximosilane of greater purity. Thecontinuous distilling-off of the carboxylic acid released during thereaction permits the reaction to continue to equilibrium. Thus, theBeckmann rearrangement is precluded, since during the process accordingto the invention, neither acids are used nor quaternary ammonium saltsformed. Thus, the proposed procedure offers a safe and economicallyattractive means of producing oximosilanes.

In the process according to the invention it is preferable to useacyloxisilanes of the above-mentioned general formula (II) in which theR¹ groups represent alkyl residues having between 1 and 8, morepreferably between 1 and 4 carbon atoms, in particular methyl or ethylresidues, alkene residues having between 2 and 8 carbon atoms, inparticular vinyl residues, if necessary phenyl-naphthyl or benzylresidues substituted singly or in multiple fashion by halogens, such asin particular chlorides or bromides, nitro groups, amino groups and/oralkyl residues having between 1 and 4 atoms, and the R⁴ groups representalkyl residues having between 2 and 8, more preferably between 2 and 4carbon atoms, in particular ethyl residues, while in the oximes havingthe general formula (III) the groups R² and R³ preferably representhydrogen atoms or alkyl residues having between 1 and 8 carbon atoms,more preferably between 1 and 4 carbon atoms, in particular methylresidues or ethyl residues.

It is especially preferable to employ in the role of acyloxisilanehaving the general formula (II), vinyl, methyl orethyltriaceto-oxisilanes that result in conjunction with usable processbyproducts. Oximes having the general formula (II) are preferablymethyl-ethyl ketoximes or dimethyl ketoximes that are also commerciallyavailable and especially well-suited for the process according to theinvention.

In the process according to the invention, the reaction should takeplace at a temperature situated preferably between 20° and 60° C., atwhich temperature the reaction runs spontaneously until chemicalequilibrium is reached. If, during the reaction, carboxylic acidreleased from the acyloxisilane of the general formula (II) is distilledoff, the reaction, as already mentioned, will proceed to 100%completion.

It is particularly advantageous for the reaction and the distillation ofthe carboxylic acid to take place under reduced pressure. It isespecially desirable if the reaction is carried out at a temperaturesituated between 30° and 120° C. and the container evacuated to apressure situated between 0.1 and 100 millibars, whereby a mixture ofoxime and released acid, preferably acetic acid, is distilled off.

In order to facilitate the reaction, the oxime having the generalformula (III) can be used in a 1 to 4M concentration.

The carboxylic acid which is distilled off during the process accordingto the invention and contains more or less large portions of the oximeused as the feedstock, can be purified in a subsequent recovery process.The resulting pure carboxylic acid and the unconverted oxime can be usedagain as feedstocks in the process according to the invention.

After the acetic acid has been distilled off in a vacuum of between 0.1and 100 millibars and at a temperature of between 50° and 80° C., anoximosilane is obtained, whose purity permits the material to be used incoloured or transparent silicon sealants. It is, however, possible topurify the oximosilane bottom through subsequent distillation at lowpressure. It is thus possible to obtain a colourless, very pureoximosilane that can be employed in all known applications.

The process according to the invention can be carried out in acontinuous, semi-continuous, or intermittent manner. Although theprocess according to the invention does not require the use of solvents,the employment of the latter, especially in the role of carrier for thecarbonic acid formed during the reaction, need not be excluded.Preferred solvents for this purpose are chlorinated aliphatic compoundssuch as perchloroethylene or trichloroethylene.

EXAMPLE

1 Mol methyl-tris(acetoxil)-silane is weighed in a 1 liter three-neckedflask equipped with a stirring device, a dripping funnel and adistilling tube with a vacuum lock. In order to ensure the completeremoval of air, the container is washed with nitrogen. The solidmethyl-tri(acetoxi)-silane is then heated to a temperature situatedbetween 40° to 60° C. and the flask is evacuated until a pressure of 1millibar is reached, whereafter for approximately 1 hour 8 mols ofmethyl-ethyl-ketoxime are added through the dripping funnel fitted witha vacuum compensator. As soon as the methyl-ethyl-ketoxime is added, thedistillation of a mixture of ketoxime and acetic acid begins.

The substitution of the acetate groups by ketoxime groups can bemonitored with the help of a gas chromatograph, whereby after thecompletion of the reaction, the excess ketoxime is removed bydistillation under vacuum at a temperature situated between 60° and 90°C. Obtained through a practically quantitative reaction relative to themethyl-tri(acetoxi)-silane employed, the end product, i.e. themethyl-tri(methyl-ethyl-ketoximo)-silane, has the form of a weak yellowyto browny coloured liquid which, during the subsequent distillationprocedure at temperatures situated between 120° and 130° C. and 1millibar, appears as a colourless liquid that still features between 6and 10% dimer or trimer components.

I claim:
 1. Process for the formation of oximosilanes of the generalformula (I)

    R.sup.1 a Si(O--N═R.sup.2 R.sup.3).sub.4-a             (I)

wherein R¹ represents a straight-chain or branched alkyl residue havingbetween 1 and 18 carbon atoms, a straight-chain or branched alkenylresidue having between 2 and 18 carbon atoms, an aryl or aralkyl residueoptionally substituted with one or more members of the group consistingof halogens, nitro groups, amino groups and alkyl residues havingbetween 1 and 4 carbon atoms, or a straight-chain or branchedalkoxy-residue having between 1 8 carbon atoms, a is an integer of 0 to3 , and R² and R³, independently of each other, represent straight-chainor branched alkyl residues having between 1 and 18 carbon atoms,optionally substituted with halogens, nitro groups, amino groups andalkyl residues having between 1 and 4 carbon atoms by a reaction of anacyloxysilane with an oxime in molar excess, comprising reacting anacyloxysilane of the general formula (II)

    R.sup.1 a Si(OCOR.sup.4).sub.4-a                           (II)

wherein R¹ and a have the above-indicated significations and R⁴represents straight-chain or branched alkyl residues having between 1and 18 carbon atoms, straight-chain or branched alkenyl residues havingbetween 2 and 18 carbon atoms or aryl or aralkyl residues which may besubstituted with halogens, nitro groups, amino groups and alkyl residueshaving between 1 and 4 carbon atoms, with a ketoxime of general formula(III) ##STR3## wherein R² and R³ have the above significations, in theabsence of an acid acceptor, while distilling off carboxylic acid,released from the acyloxysilane of the general formula (II).
 2. Aprocess according to claim 1, characterized wherein the reaction iscarried out at a temperature in the region between 20° and 60° C.
 3. Aprocess according to claim 1, wherein the reaction and thedistilling-off of the released carboxylic acid are carried out at atemperature between 30° and 120° C. and at a pressure between 0.1 and100 millibars.
 4. A process according to claim 1, wherein the oximehaving the general formula (III) is present in a 1 to 4 molar excess.